Apoptosis, or programmed cell death, is a principal mechanism by which organisms eliminate unwanted cells. The deregulation of apoptosis, either excessive apoptosis or the failure to undergo it, has been implicated in a number of diseases such as cancer, acute inflammatory and autoimmune disorders, ischemic diseases and certain neurodegenerative disorders [see generally Science, 281, pp. 1283-1312 (1998); Ellis et al., Ann. Rev. Cell. Biol., 7, p. 663 (1991)].
Caspases are a family of cysteine protease enzymes that are key mediators in the signaling pathways for apoptosis and cell disassembly [N. A. Thornberry, Chem. Biol., 5, pp. R97-R103 (1998)]. These signaling pathways vary depending on cell type and stimulus, but all apoptosis pathways appear to converge at a common effector pathway leading to proteolysis of key proteins. Caspases are involved in both the effector phase of the signaling pathway and further upstream at its initiation. The upstream caspases involved in initiation events become activated and in turn activate other caspases that are involved in the later phases of apoptosis.
The utility of caspase inhibitors to treat a variety of mammalian disease states associated with an increase in cellular apoptosis has been demonstrated using peptidic caspase inhibitors. For example, in rodent models, caspase inhibitors have been shown to reduce infarct size and inhibit cardiomyocyte apoptosis after myocardial infarction, to reduce lesion volume and neurological deficit resulting from stroke, to reduce post-traumatic apoptosis and neurological deficit in traumatic brain injury, to be effective in treating fulminant liver destruction, and to improve survival after endotoxic shock [H. Yaoita et al., Circulation, 97, pp. 276-281 (1998); M. Endres et al., J. Cerebral Blood Flow and Metabolism, 18, pp. 238-247, (1998); Y. Cheng et al., J. Clin. Invest., 101, pp. 1992-1999 (1998); A. G. Yakovlev et al., J. Neurosci., 17, pp 7415-7424 (1997); I. Rodriquez et al., J. Exp. Med., 184, pp. 2067-2072 (1996); Grobmyer et al., Mol. Med., 5, p. 585 (1999)]. However, due to their peptidic nature, such inhibitors are typically characterized by undesirable pharmacological properties, such as poor cellular penetration and cellular activity, poor oral absorption, poor stability and rapid metabolism [J. J. Plattner and D. W. Norbeck, in Drug Discovery Technologies, C. R. Clark and W. H. Moos, Eds. (Ellis Horwood, Chichester, England, 1990), pp. 92-126]. This has hampered their development into effective drugs. These and other studies with peptidic caspase inhibitors have demonstrated that an aspartic acid residue is involved in a key interaction with the caspase enzyme [K. P. Wilson et al., Nature, 370, pp. 270-275 (1994); Lazebnik et al., Nature, 371, p. 346 (1994)].
Accordingly, non-peptidyl aspartic acid mimics are useful in the synthesis of caspase inhibitors. WO96/03982 reports azaaspartic acid analogs effective as interleukin-1β converting enzyme (“ICE”) inhibitors. Fluoromethylketone analogs have also been reported as components of ICE inhibitors [WO99/47154; WO99/18781; WO93/05071].
It is well known that enzyme active sites are chiral and catalyze reactions stereospecifically. A substrate must fit precisely within this active site in order to interact with the enzyme. In accordance with this principle, a chiral compound could in many cases show improved inhibitory activity over its corresponding enantiomeric or diastereomeric mixtures.
Existing processes for synthesizing aspartic and glutamic acid derivatives and their analogues as caspase inhibitors or as intermediates for caspase inhibitors suffer from limited success and offer little control over stereochemistry [L. Revesz et al., Tetrahedron Lett., 35, pp. 9693-9696 (1994); WO91/15577; D. Rasnick, Anal. Biochem., 149, p.461 (1985)]. These transformations result in the formation of enantiomeric mixtures, which would require tedious separation steps to obtain an enantiomerically pure aspartic or glutamic acid mimic. Consequently, studies of the inhibitory activity of compounds with an aspartic or glutamic acid component generally refer to enantiomeric or diastereomeric mixtures, which may limit their effectiveness as enzyme inhibitors. Accordingly, the need exists for a process of synthesizing aspartic and glutamic acid analogues, and derivatives thereof, that are useful as intermediates for caspase inhibitors, to obtain chirally enriched derivatives in a reasonable yield.
Syntheses involving chiral diazoketones have been reported in the literature for the formation of β-amino-α-keto esters [Darkins et al., Tetrahedron Assym., 5, pp. 195-198 (1994)], N-protected allylamine derivatives [Nishi et al., Heterocycles, 29, pp. 1835-1842 (1989)], and β-homoamino acids [Ondetti et al., J. Med. Chem., 18, pp. 761-763 (1975)]. These procedures report little to no detectable racemization of the chiral center in the diazoketone transformation step.